What Is Electrical Earthing?

Definitions

Electrical Earthing (or grounding, US): an electric connections between conductive parts and local earth [defined in the IEC 60050-195-2021].

BS7671 provides the following definition: a connection of the exposed-conductive-parts of an installation to the main earthing terminal of that installation.

Protective earthing (or protective grounding, US): earthing for purposes of electrical safety [defined in the IEC 60050-195-2021].

BS7671 provides the following definition: an earthing of a point or points in a system or in an instal lation or in equipment for the purposes of safety.

Electrical Earthing Features

Connection to local earth can be: intentional, unintentional or accidental, permanent or temporary.

In other words, earthing is an action performed in an electrical installation. Therefore, it cannot, for example, be serviceable or faulty. It cannot have a resistance or any other characteristics. An earthing arrangement, for example, has a resistance. Electrical earthing can only be done or not done. This is an important point that is often misunderstood.

By electrical earthing, namely by connecting exposed-conductive-parts to protective conductors, paths are created for the flow of earth-fault currents. Protective devices must disconnect these currents when earthing is performed.

Regulatory documents establish requirements for two types of earthing: protective earthing and functional earthing.

You can see an example of how to make a protective earthing for a TT system in the figure below:

TT system 3-phase, 3-wire without the neutral conductor throughout the distribution system
Figure 1. TT system 3-phase, 3-wire without the neutral conductor throughout the distribution system

According to IEC 61140-2016, electrical earthing is not a protective measure. It is only an element of, for example, the protective measure “automatic disconnection of supply”. That is, for protection against electric shock, earthing is used in conjunction with other protective provisions. Electrical earthing alone cannot provide this protection.

Be aware that the “metal parts” of class II electrical equipment must not be earthed. The exposed-conductive-parts of class I electrical equipment must be earthed.

Another common mistake is to state that when earthing, the electric current “instantly escapes into the earth without causing any hazard to humans”. In fact, when a phase conductor is shorted to earthed conductive parts, the latter are live and hazardous to humans. In earth faults, exposed-conductive-parts in TN systems are live, usually at half the phase voltage. In a TT system this voltage can be equal to the phase voltage.

Requirement for Protective Earthing

  1. If any connection of the protective earthing is interrupted, it impairs the protection or the function of a protective measure or protective provision provided for electrical safety.
  2. Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in 411.4 to 411.6 [4].
  3. Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
  4. Conductors for protective earthing shall comply with IEC 60364-5-54.
  5. Each circuit shall have available a protective conductor connected to the relevant earthing terminal.
  6. A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed conductive-parts and suspended from such a point.

References

  1. IEC 60050-195-2021
  2. BS 7671:2018+A2:2022
  3. IEC 61140-2016
  4. IEC 60364-4-41-2017